1 What is one excellent thing to do with grass clippings, vegetable and fruit peels, and old leaves? If you guessed composting these things, you are right!2 One definition for compost from The American Heritage Science Dictionary is that it is "a mixture of decayed or decaying organic matter used to fertilize soil." When materials such as orange rinds, dead leaves, and bits of lettuce and tomatoes left over from a dinner salad are placed in a bin or pile, aerobic bacteria, fungi, and other organisms then decompose it into a useful substance that can be used as fertilizer.